fix: buffer template execution to prevent corrupt HTML responses (closes #42)

Add renderTemplate helper method on Handlers that renders templates to a
bytes.Buffer first, then writes to the ResponseWriter only on success.
This prevents partial/corrupt HTML when template execution fails partway
through.

Applied to all template rendering call sites in:
- setup.go (HandleSetupGET, renderSetupError)
- auth.go (HandleLoginGET, HandleLoginPOST error paths)
- dashboard.go (HandleDashboard)
- app.go (HandleAppNew, HandleAppCreate, HandleAppDetail, HandleAppEdit,
  HandleAppUpdate, HandleAppDeployments)

internal/handlers/render_template_test.go

@@ -0,0 +1,73 @@
+package handlers_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestRenderTemplateBuffersOutput verifies that successful template rendering
+// produces a complete HTML response (not partial/corrupt).
+func TestRenderTemplateBuffersOutput(t *testing.T) {
+	t.Parallel()
+
+	testCtx := setupTestHandlers(t)
+
+	// The setup page is simple and has no DB dependencies
+	request := httptest.NewRequest(http.MethodGet, "/setup", nil)
+	recorder := httptest.NewRecorder()
+
+	handler := testCtx.handlers.HandleSetupGET()
+	handler.ServeHTTP(recorder, request)
+
+	assert.Equal(t, http.StatusOK, recorder.Code)
+
+	body := recorder.Body.String()
+	// A properly buffered response should contain the closing </html> tag,
+	// proving the full template was rendered before being sent.
+	assert.Contains(t, body, "</html>")
+	// Should NOT contain the error text that would be appended on failure
+	assert.NotContains(t, body, "Internal Server Error")
+}
+
+// TestDashboardRenderTemplateBuffersOutput verifies the dashboard handler
+// also uses buffered template rendering.
+func TestDashboardRenderTemplateBuffersOutput(t *testing.T) {
+	t.Parallel()
+
+	testCtx := setupTestHandlers(t)
+
+	request := httptest.NewRequest(http.MethodGet, "/", nil)
+	recorder := httptest.NewRecorder()
+
+	handler := testCtx.handlers.HandleDashboard()
+	handler.ServeHTTP(recorder, request)
+
+	assert.Equal(t, http.StatusOK, recorder.Code)
+
+	body := recorder.Body.String()
+	assert.Contains(t, body, "</html>")
+	assert.NotContains(t, body, "Internal Server Error")
+}
+
+// TestLoginRenderTemplateBuffersOutput verifies the login handler
+// uses buffered template rendering.
+func TestLoginRenderTemplateBuffersOutput(t *testing.T) {
+	t.Parallel()
+
+	testCtx := setupTestHandlers(t)
+
+	request := httptest.NewRequest(http.MethodGet, "/login", nil)
+	recorder := httptest.NewRecorder()
+
+	handler := testCtx.handlers.HandleLoginGET()
+	handler.ServeHTTP(recorder, request)
+
+	assert.Equal(t, http.StatusOK, recorder.Code)
+
+	body := recorder.Body.String()
+	assert.Contains(t, body, "</html>")
+	assert.NotContains(t, body, "Internal Server Error")
+}